add 2D clustering datasets

[lindapaiste]

• Copied the 2d-cluster CSVs from the ml5 examples: https://github.com/ml5js/ml5-library/tree/main/examples/d3/KMeans/KMeans_GaussianClusterDemo/data
• Renamed the files
• Changed headings x1, x2 to x, y
• Normalized values to [0,1] so that the min and max are known
• Added a README.md and a metadata.json to explain the contents

I am not sure whether or not we want to update the ml5 examples to fetch their data from this repo rather than duplicating the files. But regardless, I thought it would be good to have the datasets here.